A white light source is generally provided by mixing light sources of different wavelengths. For example, a conventional white light source can be realized by mixing red light, green light and blue light with a suitable intensity ratio. Alternatively, the white light source can be realized by mixing yellow light and blue light with a suitable intensity ratio. The conventional method for manufacturing white light source is summarized in the following.
In a first prior art of white light source, three lED dies based on AlInGaP, InGaN and Gap are packaged into a lamp and emit red light, blue light and green light, respectively. The light emitted from the lamp can be mixed by a lens to provide white light. FIG. 1A shows a top view of a prior art white light-emitting device. The prior art white light-emitting device comprises a substrate 10, a red light-emitting diode lR, a green light-emitting diode lG and a blue light-emitting diode lB. FIG. 1B shows the emission spectrum of the prior art white light-emitting device shown in FIG. 1A. As shown in this figure, the spectrum of each light-emitting diodes is relatively narrow. However, the white light produced in this way lacks the uniform spectral distribution (especially in the 400 nm-700 nm range) of natural white light, such as sunlight. The white light thus produced has a relative chroma, which is, although indistinguishable to human eyes, differentiable to an instrument such as a camera. Therefore, the color-rendering property and reproducing ability are not satisfactory and this white light source is used mainly for lighting. Moreover, the driving voltages for lEDs of different colors are also different; this complicates the design of the driving circuit.
A second prior art of white light source was proposed by Nichia Chemical Co. in 1996, in which an InGaN based blue lED and a yellow YAG phosphor are used to provide the white light source. This white light source requires a monochrome lED to provide white light with low cost. FIG. 2 shows a top view of the second prior art white light source proposed by Nichia Chemical Co., which comprises a substrate 10, a blue light-emitting diode lB and a yellow phosphor PY coated on the blue light-emitting diode lB. The light emitted from the blue light-emitting diode lB is converted into yellow light by the yellow phosphor PY. The yellow light is mixed with the blue light to form a white light. However, the second prior art white light source proposed by Nichia Chemical Co. has a poor color-rendering property.